Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens suitable as an image pickup optical system to be used in an image pickup apparatus, such as a monitoring camera, a digital camera, a video camera, and a broadcasting camera.
Description of the Related Art
In recent years, as an image pickup optical system to be used in an image pickup apparatus, a zoom lens is required to have a high zoom ratio and a small overall system size. For example, as an image pickup optical system for a monitoring camera, the zoom lens is required to have a small overall system size and a high zoom ratio, and is also required that favorable optical characteristics can be obtained in imaging during daytime and at night.
In general, in most cases, a monitoring camera uses visible light in imaging during daytime, and uses near-infrared light in imaging at night. The use of near infrared light provides an advantage in that imaging can be carried out with less influence of scattering than when visible light is used for example, in a dense fog with low visibility. Thus, it is demanded that the zoom lens to be used in a monitoring camera be corrected for an aberration in a broad wavelength range from at visible range to a near infrared range. In addition, it is demanded that the zoom lens to be used in border surveillance or the like be, due to its applications such as monitoring a broad range and determining distant ships, a bright zoom lens that has a high zoom ratio and is favorably corrected for various aberrations across the visible range to the near-infrared range.
Hitherto, there is known a zoom lens suitable for use in a monitoring camera, which has a high zoom ratio and is corrected for various aberrations across a visible range to a near-infrared range. In each of Japanese Patent Application Laid-Open No. H02-126213 and Japanese Patent Application Laid-Open to 2009-204655, there is disclosed zoom lens having a high zoom ratio. This zoom lens includes, in order from an object side to an image side, first to fourth Lens units having positive, negative, negative, and positive refractive powers, and an interval between adjacent lens units is changed during zooming.
In the zoom lens for a monitoring camera, near-infrared light is used in most cases in imaging at night. However, there are cases where a sufficient amount of light cannot be obtained from the near-infrared light, for example, when there is very little moonlight around the time of a new moon and when the moon is hidden by a cloud. Light called nightglow (peak wavelength of 1.6 μm) is emitted when hydroxide ions in an atmosphere are excited by the sunlight. With the use of this light, favorable imaging can be achieved with ease even when there is little moonlight.
In general, near-infrared light has a feature of being less affected by scattering than visible light. Accordingly, the use of nightglow facilitates the imaging of a subject even under poor environments such as poor weather and dusty atmosphere. The nightglow, however, has a wavelength width from wavelength of 1.0 μm to a wavelength of 1.7 μm. Accordingly, when the nightglow is used for imaging, various aberrations, in particular, an axial chromatic aberration needs to be favorably corrected in a wavelength range up to near-infrared light including nightglow. With use of a zoom lens not corrected for such aberrations, high optical characteristics are not obtained.
In the zoom lenses disclosed in Japanese Patent Application Laid-Open No. H02-126213 and Japanese Patent Application Laid-Open No. 2009-204655, the zoom ratio is high and the aberrations are favorably corrected across a visible range to a near-infrared range. However, the chromatic aberration is not necessarily corrected enough up to the near-infrared range of a wavelength of 1.7 μm. Accordingly, when imaging is performed with use of nightglow, it is difficult to obtain high optical characteristics. In the zoom lens for a monitoring camera, in order to obtain favorable optical characteristics over a broad wavelength range from the visible range to the near-infrared range of a wavelength of approximately 1.7 μm while achieving a higher zoom ratio, it is important to appropriately set the zoom type and the lens configuration of each lens unit.